S7-300 Word/Bit Addressing

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Hi all,

In my recent travels, I came across the attached image, which has me a little confused. My understanding of Siemens word addressing is as follows:

MW100 contains all the bits from M100.0 to M101.7
MW101 contains all the bits from M101.0 to M102.7
Therefore, I should not use both MW100 and MW101 as they have an overlapping data area.

But this image says that MW0 contains M0.0 to M1.7, and MW1 contains M2.0 to M3.7

Which would mean that, from my example above:
MW100 contains M200.0 to M201.7
MW101 contains M202.0 to M203.7
So there is no overlap.

But the thing is, I've programmed with my original assumption before, and it worked. More than that, where I inadvertently had an overlap (e.g. used both MW100 and MW101), it didn't work.

Can anyone tell me which example is correct? :confused:

Screen Shot 2015-04-23 at 10.34.29 am.png
 
I'm not sure where that pretty drawing came from but it's wrong. The MW1 is a typo. Should be MW2. Also the bottom left side bits should be M3.0 to M3.7
If you really want to see it work put those MW's and MB's in a VAT and see
Write some values to them
 
Last edited:
Good. Thanks JRW.

The pretty drawing came from a powerpoint presentation titled "Top Ten Programming Mistakes by People New to Siemens". Go figure...
 
"[company name], a Siemens Solution Partner with the most S7 Certified Engineers in the United States, presented insights to people new to Siemens to help avoid common mistakes at the 2011 Siemens Automation Summit." (y)
 
Nope, it had an example before that saying "this is wrong". Then it prefaced that slide with "this is how Siemens addressing works"
 
It's a pity they didn't expand the example up to Byte 7. Then we could have seen from the second DW's address (MD1 or MD4) whether they were using Siemens addressing or CoDeSys and co.

Having to work with CoDeSys quite often, their addressing drives me up the wall because you can't see at a glance which Bytes (or even WORDS) are being used, whereas in Siemens it's obvious at first glamce. I've got into the habit of using an Excel Table to keep track of MB, MW and MDs in the few projects I have (mostly using V2.1) where the HMIs demand the use of the Marker area for HMI communication.
 
Last edited:

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